Composite sheet having improved adhesive properties and methods of making same

ABSTRACT

PCT No. PCT/GB94/00874 Sec. 371 Date Sep. 25, 1995 Sec. 102(e) Date Sep. 25, 1995 PCT Filed Apr. 25, 1994 PCT Pub. No. WO94/25511 PCT Pub. Date Nov. 11, 1994A composite sheet has a substrate layer, a coating layer and a flexible adhesive layer. The substrate layer is preferably a polyester film. The coating layer contains a layer mineral. The flexible adherent layer has a percentage elongation to break of greater that 150%. The composite sheet exhibits oxygen barrier properties and is suitable for use as a packaging film, particularly when laminated to a heat-sealable layer.

FIELD OF THE INVENTION

This invention relates to a composite sheet, and in particular to acomposite sheet suitable for use as a packaging material.

BACKGROUND OF THE INVENTION

There is a commercial requirement for packaging materials, especiallyfilms for packaging edible products, which exhibit barrier properties,particularly oxygen and water barrier. Existing commercially availablepackaging films consist of a multiplicity of layers, generallycomprising a metallic or polyvinylidene chloride layer to provide therequired barrier, particularly oxygen barrier, properties. However,there is presently a perceived requirement for more environmentallyacceptable materials than polyvinylidene chloride resins. In addition,coating materials which are less energy intensive than metal, such asaluminium, are required.

Coated materials comprising layer minerals are known. The layer mineralshave been used to impart a variety of properties to film substratematerials, such as flame retardancy, antistatic property and gas,particularly oxygen, barrier, as disclosed, for example in JapaneseUnexamined Patent Application No 62-181144, EP-A-235926 and U.S. Pat.No. 3,499,820.

Japanese Unexamined Patent Application No 63-233836 discloses apolymeric film having steam and gas barrier properties comprising acoating layer of a laminar silicate and a vinylidene chloride resin.

One particular type of commercially available packaging material, is afilm laminate or composite sheet comprising, in order, substrate layer(e.g. polyester film)/polyvinylidene chloride (or metal) coatinglayer/thermoset polyurethane adherent layer/heat-sealable (e.g.polyethylene) layer.

Unfortunately, when the polyvinylidene chloride (or metal) coatinglayer, in the aforementioned laminate structure is replaced by a layermineral coating layer, poor adhesion is observed between the layermineral layer and the heat-sealable layer, even when the intermediatethermoset polyurethane adherent layer is employed.

SUMMARY OF THE INVENTION

We have now devised a composite sheet comprising a layer mineral coatinglayer which can exhibit barrier, particularly to oxygen, properties andimproved adhesion to a subsequently applied heat-sealable layer.

Accordingly, the present invention provides a composite sheet comprisinga substrate layer having on at least one surface thereof a coating layercomprising a layer mineral, the remote surface of said coating layerhaving thereon a flexible adherent layer having a percentage elongationto break of greater than 150%.

The invention also provides a method of producing a composite sheet byforming a substrate layer, applying to at least one surface thereof acoating layer comprising a layer mineral, and applying to the remotesurface of said coating layer a flexible adherent layer having apercentage elongation to break of greater than 150%.

DETAILED DESCRIPTION OF THE INVENTION

A substrate layer for use in the production of a composite sheetaccording to the invention suitably comprises any film or web material,such as paper, paperboard, or synthetic paper. In a preferred embodimentof the invention the substrate is formed from polymeric material, whichis preferably capable of forming a self-supporting opaque, or preferablytransparent, film or sheet.

By a "self-supporting film or sheet" is meant a film or sheet capable ofindependent existence in the absence of a supporting base.

The substrate layer of a composite sheet according to the invention maybe formed from any film-forming, polymeric material. Suitablethermoplastics, synthetic, materials include a homopolymer or acopolymer of a 1-olefine, such as ethylene, propylene or butene-1,especially polypropylene, a polyamide, a polycarbonate, and particularlya synthetic linear polyester which may be obtained by condensing one ormore dicarboxylic acids or their lower alkyl (up to 6 carbon atoms)diesters, e.g. terephthalic acid, isophthalic acid, phthalic acid, 2,5-,2,6- or 2,7-naphthalenedicarboxylic acid, succinic acid, sebacic acid,adipic acid, azelaic acid, 4,4'-diphenyldicarboxylic acid,hexahydro-terephthalic acid or 1,2-bis-p-carboxyphenoxyethane(optionally with a monocarboxylic acid, such as pivalic acid) with oneor more glycols, particularly an aliphatic glycol, e.g. ethylene glycol,1,3-propanediol, 1,4-butanediol, neopentyl glycol and1,4-cyclohexanedimethanol. A polyethylene terephthalate or polyethylenenaphthalate film is preferred. A polyethylene terephthalate film isparticularly preferred, especially such a film which has been biaxiallyoriented by sequential stretching in two mutually perpendiculardirections, typically at a temperature in the range 70° to 125° C., andpreferably heat set, typically at a temperature in the range 150° to250° C., for example--as described in British patent 838,708.

The substrate may also comprise a polyarylether or thio analoguethereof, particularly a polyaryletherketone, polyarylethersulphone,polyaryletheretherketone, polyaryletherethersulphone, or a copolymer orthioanalogue thereof. Examples of these polymers are disclosed inEP-A-1879, EP-A-184458 and U.S. Pat. No. 4,008,203. The substrate maycomprise a poly(arylene sulphide), particularly poly-p-phenylenesulphide or copolymers thereof. Blends of the aforementioned polymersmay also be employed.

Suitable thermoset resin substrate materials includeaddition--polymerisation resins--such as acrylics, vinyls,bis-maleimides and unsaturated polyesters, formaldehyde condensateresins--such as condensates with urea, melamine or phenols, cyanateresins, functionalised polyesters, polyamides or polyimides.

The preferred polymeric film substrate for production of a compositesheet according to the invention may be unoriented, or uniaxiallyoriented, but is preferably biaxially oriented by drawing in twomutually perpendicular directions in the plane of the film to achieve asatisfactory combination of mechanical and physical properties.Simultaneous biaxial orientation may be effected by extruding athermoplastics polymeric tube which is subsequently quenched, reheatedand then expanded by internal gas pressure to induce transverseorientation, and withdrawn at a rate which will induce longitudinalorientation. Sequential stretching may be effected in a stenter processby extruding the thermoplastics substrate material as a flat extrudatewhich is subsequently stretched first in one direction and then in theother mutually perpendicular direction. Generally, it is preferred tostretch firstly in the longitudinal direction, i.e. the forwarddirection through the film stretching machine, and then in thetransverse direction. A stretched substrate film may be, and preferablyis, dimensionally stabilised by heat-setting under dimensional restraintat a temperature above the glass transition temperature thereof.

The substrate is suitably of a thickness from 6 to 300, particularlyfrom 6 to 100, and especially from 6 to 25 μm.

The layer mineral preferably comprises platelets of a film-forming, 2:1phyllosilicate layer mineral. For information on the composition andstructure of phyllosilicate layer minerals, reference can be made to"Clay Minerals: Their Structure, Behaviour & Use", Proceedings of aRoyal Society Discussion Meeting, 9 & 10 Nov. 1983, London, The RoyalSociety, 1984 (particularly pages 222-223, 232-235).

The term "platelets" as used in this specification means tiny particlesof the layer mineral obtained by subjecting the mineral to a chemicaldelaminating process to form an aqueous colloidal dispersion of highaspect ratio particles of the mineral from which a film can be formed.

Preferably, the layer mineral is selected from the group consisting ofsmectites, preferably hectorits and montmorillonite, and particularlyvermiculite.

The term "vermiculite" as used in this specification means all materialsknown mineralogically and commercially as vermiculite. Vermiculite orebeing a naturally-occuring mineral contains a mixture of phases (e.g.vermiculite, biotite, hydrobiotite etc) and a mixture of interlayercations (e.g. Mg²⁺, Ca²⁺, K⁺). Production of aqueous suspensions orslurries of vermiculite platelets rely on ion exchange (normallyincomplete) to generate adequate macroscopic swelling. The swollen,fully- or partially-exchanged vermiculite gel can then be milled toproduce a film-forming aqueous suspension of vermiculite platelets.Treatment of vermiculite particles with one or more aqueous solutions ofmetal (especially alkali metal) salts or alkyl ammonium salts followedby swelling in water and then milling to delaminate the vermiculite iswell known and is described for example in GB-A-1016385, GB-A-1119305,GB-A-1585104 and GB-A-1593382, and in U.S. Pat. No. 4,130,687.

Vermiculite is a particularly suitable layer mineral when a compositesheet exhibiting gas barrier, especially oxygen barrier, is required. Acomposite sheet according to the invention suitably has an oxygenpermeability of less than 50, preferably less than 20, more preferablyless than 10, particularly less than 5, and especially less than 1 cc/m²/day.

A preferred embodiment of the invention comprises a coating layer ofvermiculite platelets wherein greater than 50%, preferably 55 to 99.9%,more preferably 60 to 99%, and particularly 70 to 95% by number of theplatelets have a particle size (by which is meant the size of themaximum width of a platelet) in the range 0.5 to 5.0 μm. It is alsopreferred that 80 to 99.9%, more preferably 85 to 99.9%, and especially90 to 99.9% by number of the vermiculite platelets have a particle sizein the range 0.1 to 5.0 μm. The mean particle size (by which is meantthe mean value of the maximum width of the platelets) of vermiculiteplatelets is preferably 1.0 to 3.0 μm, more preferably 1.2 to 2.2 μm,and especially 1.3 to 1.6 μm. It is also preferred that the vermiculiteplatelets have a thickness in the range from approximately 10 to 60 A,especially from approximately 25 to 40 A. In addition, it is preferredthat from 60 to 100%, more preferably from 70 to 99%, and particularlyfrom 90 to 95% by number of vermiculite platelets have a thickness inthe range 10 to 60 A. The mean thickness of vermiculite platelets ispreferably 25 to 50 A, more preferably 25 to 40 A, and especially 25 to30 A.

Although the coating layer can comprise a substantially continuous layerof platelets of any practical thickness, suitably up to 5 μm, preferablyup to 2 μm, and more preferably up to 0.5 μm, composite sheetsexhibiting desired properties, for example improved barrier propertyagainst atmospheric oxygen, comprise a substantially continuous layer ofplatelets at very low thicknesses, e.g. as low as 0.01 μm, especially inthe range from 0.02 μm to 0.3 μm, and particularly in the range from 0.1μm to 0.25 μm.

In a preferred embodiment of the invention the coating layeradditionally comprises at least one material which may be any polymerknown in the art to be capable of forming a continuous, preferablyuniform coating. The polymeric material is preferably an organic resinand may be any film-forming polymeric or oligomeric species or precursortherefor that assists in forming a film of the layer mineral and doesnot disrupt the film-forming capability thereof.

Suitable polymeric resins include:

(a) "aminoplast" resins which can be prepared by the interaction of anamine or amide with an aldehyde, typically an alkoxylated condensationproduct of melamine and formaldehyde, e.g. hexamethoxymethylmelamine,trimethoxy trimethylol melamine formaldehyde;

(b) homopolyesters, such as polyethylene terephthlate;

(c) copolyesters, particularly those derived from a sulpho derivative ofa dicarboxylic acid such as sulphoterephthalic acid and/orsulphoisophthalic acid;

(d) copolymers of styrene with one or more ethylenically unsaturatedcomonomers such as maleic anhydride or itaconic acid, especially thecopolymers described in GB-A-1540067;

(e) copolymers of acrylic acid and/or methacrylic acid and;or theirlower alkyl (up to 6 carbon atoms) esters, e.g. copolymers of ethylacrylate and methyl methacrylate, copolymers of methylmethacrylate/buryl acrylate/acrylic acid typically in the molarproportions 55/27/18% and 36/24/40%;

(f) copolymers of styrene/acrylamide, particularly of the type describedin GB-A-1174328 and GB-A-1134876;

(g) functionalised polyolefins, especially maleinised polybutadiene;

(h) cellulosic materials such as nitrocellulose, ethylcellulose andhydroxyethylcellulose:

(i) polyvinyl alcohol; and

(j) polyurethane resins.

A suitable polymeric resin component of the coating layer comprises anacrylic or methacrylic resin, preferably thermoset and preferablycomprising a polymer comprising at least one monomer derived from anester of acrylic acid and/or an ester of methacrylic acid, especially analkyl ester of (meth)acrylic acid where the alkyl group contains up toten carbon atoms such as methyl, ethyl, n-propyl, isopropyl, n-butyl,isobutyl, terbutyl, hexyl, 2-ethylhexyl, heptyl, and n-octyl. Theacrylic resin preferably comprises greater than 50 mole %, andpreferably less than 98 mole %, especially from 70 to 96 mole %, andparticularly from 80 to 94 mole % of at least one monomer derived froman ester of acrylic acid and/or an ester of methacrylic acid, and/orderivatives thereof. Polymers derived from an alkyl acrylate, forexample ethyl acrylate and butyl acrylate, together with an alkylmethacrylate are preferred. Polymers comprising ethyl acrylate andmethyl methacrylate are particularly preferred The acrylate monomer ispreferably present in a proportion in the range 30 to 65 mole %, and themethacrylate monomer is preferably present in a proportion in the rangeof 20 to 60 mole %.

A preferred acrylic resin, derived from 3 monomers comprises 35 to 60mole % of ethyl acrylate/30 to 55 mole % of methyl methacrylate/2-20mole % of methacrylamide, and especially comprising approximate molarproportions 46/46/8% respectively of ethyl acrylate/methylmethacrylate/acrylamide or methacrylamide, the latter polymer beingparticularly effective when thermoset for example, in the presence ofabout 25 weight % of a methylated melamine-formaldehyde resin. Theacrylic resin is preferably present in the coating layer in the rangefrom 0.5 to 50% by weight of the coating layer.

In a particularly preferred embodiment of the invention the coatinglayer is formed from a composition comprising a cross-linking agent, bywhich is meant a material which reacts chemically to form a polymericresin during formation of the coating layer, preferably forming covalentbonds, both with itself and with the surface of the underlying layer toform cross-links thereby improving adhesion thereto. The cross-linkingagent is suitably an organic material, preferably a monomeric and/oroligomeric species, and particularly monomeric, prior to formation ofthe coating layer. The molecular weight of the cross-linking agent ispreferably less than 5000, more preferably less than 2000, especiallyless than 1000, and particularly in the rates from 250 to 500.Additionally, the cross-linking agent should preferably be capable ofinternal cross-linking in order to provide protection against solventpenetration. Suitable cross-linking agents may comprise epoxy resins,alkyd resins, amine derivatives such as hexamethoxymethyl melamine,and/or condensation products of an amine, e.g. melamine, diazins, urea,cyclic ethylene urea, cyclic propylene urea, thiourea, cyclic ethylenethiourea, aziridines, alkyl melamines, aryl melamines, benzo guanamines,guanamines, alkyl guanamines and aryl guanamines, with an aldehyde, e.g.formaldehyde. A preferred cross-linking agent is the condensationproduct of melamine with formaldehyde. The condensation product mayoptionally be alkoxylated. A catalyst is also preferably employed tofacilitate cross-linking action of the cross-linking agent. Preferredcatalysts for cross-linking melamine formaldehyde include para toluenesulphonic acid, maleic acid stabilised by reaction with a base, andmorpholinium paratoluene sulphonate.

The coating layer preferably comprises treater than 20% and less than80%, more preferably greater than 25% and less than 70%, particularlytreater than 30% and less than 60%, and especially treater than 30% andless than 50% by weight of the layer of the resin derived bycross-linking the cross-linking agent. The coating layer preferablycomprises greater than 20% and less than 80%, more preferably greaterthan 30% and less than 75%, particularly greater than 40% and less than70%, and especially greater than 50% and less than 70% by weight of thelayer of the layer mineral.

In a further embodiment of the invention the coating layer mayadditionally comprise a copolymer of a 1-olefine with a vinyl monomer.Suitable 1-olefines include ethylene, propylene and butene-1. Ethyleneis particularly preferred. The vinyl monomer is preferably a vinylester, such as vinyl chloroacetate, vinyl benzoate, and particularlyvinyl acetate. An ethylene/vinyl acetate copolymer is especiallypreferred. The ratio of the amount of 1-olefine to vinyl monomer presentin the copolymer is preferably in the range from 5:1 to 1:5, morepreferably 4:1 to 1:2, and especially 3:1 to 1:1. The amount of thecopolymer of a 1-olefine with a vinyl monomer is preferably present inthe coating layer within the range from 1% to 30%, more preferably from2% to 20%, particularly from 2.5% to 15%, and especially from 2.3% to 4%by weight of the layer. The copolymer of a 1-olefine with a vinylmonomer is preferably mixed with the layer mineral, prior to adding thecross-linking agent to the coating composition.

The coating layer is formed by applying the coating composition,comprising the layer mineral, and preferably the cross-linking agent, asa slurry or dispersion and removing the dispersion medium to form acoherent layer. Preferably, the dispersion medium comprises water andthe slurry or dispersion comprises between 0.5% and 20%, more especiallybetween 1% and 10%, by weight of the layer mineral.

The coating layer composition may be applied before, during or after thestretching operation in the production of an oriented film. The coatinglayer composition may, for example, be applied to the film substratebetween the two stages (longitudinal and transverse) of a thermoplasticsfilm biaxial stretching operation. Such a sequence of stretching andcoating may be suitable for the production of a coated linear polyesterfilm substrate, which is preferably firstly stretched in thelongitudinal direction over a series of rotating rollers, coated, andthen stretched transversely in a stenter oven, preferably followed byheat setting. The coating composition is preferably applied to analready oriented film substrate, such as a biaxially oriented polyester,particularly polyethylene terephthalate film, which is preferablysubjected to heating.

The temperature to which the coated film is heated depends, inter aliaon the composition of the substrate layer. A coated polyester,especially polyethylene terephthalate, substrate is suitably heated from100° C. to 240° C., preferably from 150° C. to 180° C., in order to drythe aqueous medium, or the solvent in the case of solvent-appliedcompositions, and for cross-linking of the layer to occur and also toassist in coalescing and forming the coating into a continuous anduniform layer. In contrast, a coated polyolefin, especiallypolypropylene, is suitably heated in the range 85° C. to 95° C.

The coating layer composition is preferably applied to the substrate byany suitable conventional technique such as gravure coating, dipcoating, bead coating, reverse roller coating or slot coating.

The coating layer is preferably applied to the substrate at a dry coatweight within the range 0.25 to 50 mgdm⁻², more preferably 0.5 to 20mgdm⁻², especially 1.0 to 5.0 mgdm⁻². For substrates coated on bothsurfaces, each layer preferably has a coat weight within the preferredrange.

The flexible adherent layer preferably comprises a thermoplastic orrubbery polymeric resin material, and is preferably not thermoset. Theadherent layer preferably exhibits a percentage elongation to break(/ETB) in the range from 300% to 10,000%, more preferably in the rangefrom 600% to 4000%, particularly in the range from 800% to 2000%, andespecially in the range from 1200% to 1700%.

In a preferred embodiment of the invention the adherent layer has atensile modulus (1% secant modulus) of less than 2.0 MPa, preferably inthe range from 0.05 MPa to 1.5 MPa, more preferably in the range from0.1 MPa to 1.0 MPa, particularly in the range from 0.2 MPa to 0.7 MPa,and especially in the range from 0.25 MPa to 0.45 MPa.

The chemical composition of the polymeric resin of the adherent layermay vary over a relatively wide range of materials. It is the physicalproperties of the adherent layer; rather than the precise chemicalcomposition which, we believe, is responsible for the surprisinglyimproved properties of a composite sheet according to the invention, asdescribed herein.

Suitable adherent layer polymeric resins are natural and syntheticrubbers, e.g. where 1,4-polyisoprene is a major component. Otherpreferred polymeric resins include polyterpene resins,isoprene-piperylene copolymers, styrene-butadiene copolymers,styrene-isoprene block copolymers, rubbery branched or radial blockcopolymers of conjugated dienes and vinyl aromatic hydrocarbons, cyclicmonoolefin copolymers, carboxyl-containing chloroprene polymers,ethylene-vinyl acetate-diolefin-ester copolymers, polyisocyanatemodified natural or synthetic rubber, and non-thermoset polyurethaneresins.

In a preferred embodiment of the invention, the polymeric resin of theadherent layer is a styrene-butadiene copolymer, preferably a blockcopolymer. The molar ratio of styrene:butadiene is preferably in therange from 0.1 to 10:1, more preferably in the range from 0.5 to 3:1,and especially in the range from 1.2 to 1.6:1.

The molecular weight of polymeric resin of the adherent layer can varyover a wide range but the weight average molecular weight is preferablywithin the range 5,000 to 1,000,000, more preferably within the range10,000 to 500,000, and especially within the range 20,000 to 300,000.

The adherent layer coating composition may comprise other materials usedto modify the properties of the resultant adherent layer, such asantioxidants, plasticisers, and tackifying resins.

The polymer(s) of the adherent layer coating composition may bewater-soluble or water-insoluble. The adherent layer compositionincluding any water-insoluble polymer may nevertheless be applied to thecoating layer surface as an aqueous dispersion or alternatively as asolution in an organic solvent.

The adherent layer composition is preferably applied onto a driedcoating layer by any suitable conventional technique such as dipcoating, bead coating, reverse roller coating or slot coating.

The adherent layer is preferably dried by heating from 70° C. to 160°C., preferably from 80° C. to 100° C., in order to dry the aqueousmedium, or the solvent in the case of solvent-applied compositions, andto assist in coalescing and forming the adherent composition into acontinuous and uniform layer.

The adherent layer is preferably applied to the coating layer at a coatweight within the range 10 to 200 mgdm⁻², more preferably 20 to 150mgdm⁻², and especially 50 to 100 mgdm⁻².

The thickness of the dried adherent layer is preferably in the rangefrom 1 μm to 20 μm, more preferably 2 μm to 15 μm, and especially 5 μmto 10 μm.

In one embodiment of the invention, an additional primer layer isapplied to the substrate layer prior to coating with the layer mineralcoating layer. The presence of the primer layer can result in animprovement in the adhesion of the coating layer to the substrate and/oran improvement in barrier properties, particularly oxygen barrier.

The primer layer comprises at least one polymeric resin which may be anypolymer known in the art to be capable of forming a continuous,preferably uniform, coating, being adherent to the supporting substrate,and preferably exhibiting optical clarity. Polymeric resins which aresuitable for forming the primer layer include all the polymeric resinsdisclosed herein for use in the coating layer.

A preferred polymeric resin component of the primer layer comprises acopolymer comprising acrylamide and/or a derivative thereof, such as alower alkoxy, preferably n-butoxy, and/or methacrylamide and/or aderivative thereof, such as a lower alkoxy, preferably n-butoxy, and atleast one other ethylenically unsaturated comonomer copolymerisabletherewith, including acrylic acid and its esters, including alkylesters, for example methyl acrylate, ethyl acrylate, butyl acrylate,2-ethyl-hexyl acrylate, isobutyl acrylates, hexyl acrylates and octylacrylates; methacrylic acid and its esters for example methylmethacrylate, ethyl methacrylate and butyl methacrylate. Other suitablemonomers include acrylonitrile, styrene, monomethyl styrene, vinyltoluene, maleic anhydride, and vinyl ethers. Styrene and alkyl acrylatesare particularly preferred monomers. Dienes such as butadiene orchloroprene may also be present in the primer layer copolymer.

The primer layer copolymer preferably comprises at least one freefunctional acid (for example a pendant carboxyl or sulphonate group)group, that is a group other than those involved in the polymerisationreaction by which the copolymer is formed, such as acrylic acid,methacrylic acid, maleic acid and itaconic acid. Suitably up to 25%,preferably up to 10%, and especially up to 5% by weight of comonomercontaining a free functional acid group may be employed.

The polymeric resin component of the primer layer preferably comprises acopolymer derived from up to 90% by weight of styrene, up to 80% byweight of an alkyl acrylate, up to by weight of methacrylic acid, andfrom 5% to 40% by weight of acrylamide which has been condensed with asolution of formaldehyde in n-butanol containing from 0.2 to 3equivalents of formaldehyde for each amide group in the copolymer. Aparticularly preferred copolymer comprises styrene/2-ethyl hexylacrylate/methacrylic acid/n-butoxymethyl acrylamide in a ratio of 20 to40/30 to 50/1 to 5/20 to 35% by weight. GB-A-1174328 and GB-A-1134876describe suitable methods for the synthesis of the aforementioned primerlayer copolymers.

The polymeric resin of the primer layer is preferably compatible with,and includes, an additional acidic component, such as sulphuric, nitric,acetic acid or any mineral acid, such as hydrochloric acid, in theprimer layer coating composition. The additional acidic component ispreferably present in the primer layer coating composition in aconcentration range from 0.5 to 15, more preferably 1 to 10, andespecially 4 to 8% by weight relative to the weight of the polymericresin. Sulphuric acid is a preferred acidic component.

In an alternative embodiment of the invention the polymeric resincomponent of the primer layer comprises the acrylic or methacrylic resindescribed hereinbefore as a suitable polymeric resin component of thecoating layer. A preferred acrylic resin comprises 35 to 60 mole % ofethyl acrylate/30 to 55 mole % of methyl methacrylate/2-20 mole % ofmethacrylamide, and especially comprising approximate molar proportions46/46/82% respectively of ethyl acrylate/methyl methacrylate/acrylamideor methacrylamide, the latter polymer being particularly effective whenthermoset for example, in the presence of about 25 weight % of amethylated melamine-formaldehyde resin.

The molecular weight of polymeric resin of the primer layer can varyover a wide range but the weight average molecular weight is preferablywithin the range 10,000 to 300,000, and more preferably within the range15,000 to 100,000.

The polymer(s) of the primer layer coating composition is generallywater-insoluble. The primer layer composition including thewater-insoluble polymer may nevertheless be applied to the substrate asan aqueous dispersion or alternatively as a solution in an organicsolvent.

The primer layer coating medium my be applied before, during or afterany stretching operation used in the production of a polymericsubstrate. In particular, the coating medium my be applied to thepolymeric film substrate between the two stages (longitudinal andtransverse) of a biaxial stretching operation. Such a sequence ofstretching and coating is suitable for the production of a coated linearpolyester film substrate, such as a coated polyethylene terephthalatefilm, which is preferably firstly stretched in the longitudinaldirection over a series of rotating rollers, coated, and then stretchedtransversely in a stenter oven, preferably followed by heat setting.

The primer layer coating composition may be applied to the substrate asan aqueous dispersion or solution in an organic solvent by any suitableconventional coating technique such as dip coating, bead coating,reverse roller coating or slot coating.

The primer layer coating medium is preferably applied to an alreadyoriented polymeric film substrate, such as a biaxially orientedpolyester, particularly polyethylene terephthalate film.

Prior to deposition of the primer layer or coating layer onto thesubstrate, the exposed surface thereof may, if desired, be subjected toa chemical or physical surface-modifying treatment to improve the bondbetween that surface and the subsequently applied primer or coatinglayer. A preferred treatment, because of its simplicity andeffectiveness, which is particularly suitable for the treatment of apolyolefin substrate, is to subject the exposed surface of the substrateto a high voltage electrical stress accompanied by corona discharge.Alternatively, the substrate may be pretreated with an agent known inthe art to have a solvent or swelling action on the substrate polymer.Examples of such agents, which are particularly suitable for thetreatment of a polyester substrate, include a halogenated phenoldissolved in a common organic solvent e.g. a solution ofp-chloro-m-cresol, 2,4-dichlorophenol, 2,4,5- or 2,4,6-trichlorophenolor 4-chlororesorcinol in acetone or methanol.

The primer layer is preferably applied to the substrate at a coat weightwithin the range 0.1 to 10 mgdm⁻², especially 1.0 to 6 mgdm⁻². Forsubstrates coated on both surfaces, each layer preferably has a coatweight within the preferred range.

Modification of the surface of the primer layer, e.g. by flametreatment, ion bombardment, electron beam treatment, ultra-violet lighttreatment or preferably by corona discharge, may improve the adhesion ofthe subsequently applied coating layer comprising a layer mineral, butmay not be essential to the provision of satisfactory adhesion.

The preferred treatment by corona discharge may be effected in air atatmospheric pressure with conventional equipment using a high frequency,high voltage generator, preferably having a power output of from 1 to 20kw at a potential of 1 to 100 kv. Discharge is conveniently accomplishedby passing the film over a dielectric support roller at the dischargestation at a linear speed preferably of 1.0 to 500 m per minute. Thedischarge electrodes may be positioned 0.1 to 10.0 mm from the movingfilm surface.

The ratio of substrate to primer layer thickness may vary within a widerange, although the thickness of the primer layer preferably should notbe less than 0004% nor greater than 10% of that of the substrate. Inpractice, the thickness of the primer layer is desirably at least 0.005μm and preferably should not greatly exceed about 1.0 μm.

A composite sheet according to the invention is suitably coated with, orlaminated to, a heat-sealable layer in order to form a packaging filmlaminate. The heat-sealable layer comprises polymeric material whichpreferably should be capable of forming a heat-seal bond to itself or tothe substrate, or preferably to both, by heating to soften the polymericmaterial of the heat-sealable layer and applying pressure withoutsoftening or melting the material of the substrate layer. Theheat-sealable layer suitably has a heat-seal strength to itself ofgreater than 500 g/25 mm (196 Nm⁻¹), preferably in the range from 800 to5000 g/25 mm (314 to 1960 Nm⁻¹), more preferably 1000 to 4000 g/25 mm(392 to 1568 Nm⁻¹), and particularly 1500 to 3000 g/25 mm (588 to 1176Nm⁻¹). The heat-seal strength can be measured by sealing theheat-sealable layer to itself at 140° C. for 1 second under a pressureof 103 kPa (15 psi), cooling to room temperature, and measuring theforce required under linear tension per unit width of seal to peel thesealed films apart at a constant speed of 4.23 mm/second.

A heat-sealable layer suitably comprises a polyester or polyolefinresin. Suitable polyesters include copolyester resins, especially thosederived from one or more dibasic aromatic carboxylic acids, such asterephthalic acid, isophthalic acid and hexahydroterephthalic acid, andone or more glycols, such as ethylene glycol, diethylene glycol,triethylene glycol and neopentyl glycol. Typical copolyesters whichprovide satisfactory heat-sealable properties are those of ethyleneterephthalate and ethylene isophthalate, especially in the molar ratiosof from 50 to 90 mole % ethylene terephthalate and correspondingly from50 to 10 mole % ethylene isophthalate.

A polyolefin resin, particularly polyethylene is a preferred componentof the heat-sealable layer.

Formation of a heat-sealable layer on the adherent layer may be effectedby applying a polymeric resin as an aqueous dispersion or solution in anorganic solvent, by any suitable conventional coating technique such asdip coating, bead coating, reverse roller coating or slot coating.Alternatively, the heat-sealable layer may be extrusion coated.Preferably a preformed heat-sealable layer film, particularly apolyethylene film, is laminated to a composite sheet according to theinvention, by passing through a heated nip roll.

The heat-sealable layer preferably has a thickness of up to 100 μm, morepreferably greater than 10 μm, and especially from 35 μm to 70 μm.

One or more of the layers of a composite sheet according to theinvention, i.e. substrate, primer, coating, adherent and/orheat-sealable layer(s), may conveniently contain any of the additivesconventionally employed in the manufacture of polymeric films. Thus,agents such as dyes, pigments, voiding agents, lubricants,anti-oxidants, anti-blocking agents, surface active agents, slip aids,gloss-improvers, prodegradants, ultra-violet light stabilisers,viscosity modifiers and dispersion stabilisers may be incorporated intoone or more of the aforementioned layers as appropriate. In particular,a substrate may comprise a particulate filler, such as silica, of smallparticle size. Desirably, a filler, if employed in a substrate layer,should be present in a small amount not exceeding 0.5% preferably lessthan 0.2%, by weight of the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated by reference to the accompanying drawingsin which:

FIG. 1 is a schematic sectional elevation, not to scale, of a compositesheet having a substrate, coating and adherent layers.

FIG. 2 is a similar schematic elevation of a film as shown in FIG. 1,with an additional heat-sealable layer on the remote surface of theadherent layer.

FIG. 3 is a similar schematic elevation of a film as shown in FIG. 2,with an additional primer layer between the substrate and coatinglayers.

Referring to FIG. 1 of the drawings, the film comprises a substratelayer (1) having a coating layer (2) bonded to one surface (3) thereof.An adherent layer (4) is bonded to the remote surface (5) of the coatinglayer (2).

The film of FIG. 2 further comprises an additional heat-sealable layer(6), bonded to the remote surface (7) of the adherent layer (4).

The film of FIG. 2 further comprises an additional primer layer (8),bonded to the surface (3) of the substrate (1) and the surface (9) ofthe coating layer (2).

The invention is further illustrated by reference to the followingexamples.

EXAMPLE 1

A vermiculite suspension was prepared in accordance with the followingmethod:

100 g of vermiculite ore (ex-Carolina, U.S.) was refluxed with 1 literof 1N lithium citrate solution at 80° C. for four hours and left at roomtemperature for five days. This resulted in the exchange of 45.3 mEq/100g of lithium cations in the ore. The ion-exchanged vermiculite ore waswashed 5 times with deionised water to remove the excess salts and leftovernight to swell in a large volume (i.e. 10 times the volume ofvermiculite) of deionised water. The swollen vermiculite was milled for40 minutes in a Greaves high-shear mixer at 7200 rpm to produce asuspension of vermiculite platelets. The platelet suspension was thensieved by passing through a mechanically agitated filter having a poresize of 106 μm.

A polyethylene terephthalate film was melt extruded, cast onto a cooledrotating drum and stretched in the direction of extrusion toapproximately 3 times its original dimensions. The film was passed intoa stenter oven, where the film was stretched in the sideways directionto approximately 3 times its original dimensions. The biaxiallystretched film was heat set at a temperature of about 220° C. byconventional means. Final film thickness was 23 μm.

The biaxially oriented polyethylene terephthalate film was coated on oneside using a gravure coater, with a coating layer composition comprisingthe following ingredients:

    ______________________________________                                        Vermiculite               6      l                                            (7.5% w/w aqueous dispersion)                                                 Cymel 385                 375    ml                                           (melamine formaldehyde of molecular weight 348)                               Para toluene sulphonic acid                                                                             216    ml                                           (10% w/w aqueous solution)                                                    Synperonic NP 10          100    ml                                           (10% w/w aqueous solution of a nonyl phenol                                   ethoxylate, supplied by ICI)                                                  Demineralised water       to 10  liters                                       ______________________________________                                    

The coated film was dried at 180° C., and the dry coat weight of thecoating layer was approximately 2 mgdm⁻². The thickness of the coatinglayer was approximately 0.2 μm.

A composite sheet was formed by coating the remote surface of the layermineral coating layer, using a gravure coater, with an adherent layercomposition comprising the following ingredient:

Rubber based resin (Product No 10-2060, supplied by Holden SurfaceCoatings Ltd, England)

The adherent layer coated film was dried at 90° C., and the dry coatweight of the adherent layer was approximately 100 mgdm⁻². The thicknessof the adherent layer was approximately 8 μm.

The tensile modulus and ZETB of the adherent layer were measured byfirst of all casting, onto a release film, a sample of the adherentlayer composition. After drying, the sample of adherent layer was peeledaway from the release film, and used to measure the tensile modulus andZETB. The tensile modulus 1% secant modulus) and ZETB of the adherentlayer sample (30 mm long, 15 mm wide and 2 mm thick) were measured usingan Instron Model 1122 Universal Testing Machine at 23° C. and 50%relative humidity. A displacement rate of 10 mm min⁻¹ was used tomeasure the tensile modulus, and a displacement rate of 50 mm min⁻¹ wasused to measure the ZETB.

The adherent layer sample exhibited a tensile modulus of 0.35 MPa and aZETB of 1552%.

The oxygen barrier property of the composite sheet(substrate/coating/adherent layers) was determined by measuring theoxygen permeability using a Mocon 1050 (Modern Controls Inc.) testinstrument. A sample of the composite sheet was placed in the machinewith the nitrogen carrier gas (containing 1% hydrogen) flowing bothabove and below the sheet, in order that a background reading can beobtained. The nitrogen above the sheet was replaced by oxygen, and theamount of oxygen able to permeate through the sheet was measured in acarrier gas by using a sensor. The oxygen permeability was 1.5 cc/m²/day. The oxygen permeability of uncoated polyethylene terephthalatefilm was also measured and found to be 50.0 cc/m² /day.

The strength of adhesion of the composite sheet to a heat-sealable layerwas measured by laminating the adherent layer surface of the compositesheet with a 50 μm polyethylene film, by passing the two films through anip heated at 70° C. The adhesion was measured by peeling apart theresulting laminate using an "Instron" Tensometer at a displacement rateof 50 mm min⁻¹. The peel strength was 220 g/25 mm (86 Nm⁻¹).

EXAMPLE 2

This is a comparative example not according to the invention. Theprocedure of Example 1 was repeated except that the adherent layercomposition comprised the following ingredients:

    ______________________________________                                        Polyurethane prepolymer  69    parts                                          (Product No 10-2525/3, supplied by                                            Holden Surface Coatings Ltd, England)                                         Polyhydric alcohol       1     part                                           (Product No 10-2526/3 supplied by                                             Holden Surface Coatings Ltd, England)                                         ______________________________________                                    

The adherent layer coated film was dried at 90° C., and the dry coatweight of the adherent layer was approximately 30 mgdm⁻². The thicknessof the adherent layer was approximately 3 μm.

The tensile modulus and ZETB of the adherent layer were measured asdescribed in Example 1. The adherent layer sample exhibited a tensilemodulus of 2.3 MPa and a % ETB of 91.5%.

The oxygen barrier property of the composite sheet was determined asdescribed in Example 1, and the oxygen permeability was 1.5 cc/m² /day.

The strength of adhesion of the composite sheet to a heat-sealable layerwas also determined as described in Example 1, and the peel strength wasless than 10 g/25 mm (less than 4 Nm⁻¹).

The above examples illustrate the improved properties of a compositesheet according to the present invention.

We claim:
 1. A composite sheet comprising a substrate layer having on atleast one surface thereof a coating layer comprising a layer mineral,the remote surface of said coating layer having thereon a flexibleadherent layer having a percentage elongation to break of greater than150% and a tensile modulus (1% secant modulus) of less than 2.0 MPa. 2.A composite film according to claim 1 wherein the adherent layer has atensile modulus (1% secant modulus) in the range from 0.05 to 1.5 MPa.3. A composite sheet according to claim 2 wherein the adherent layer hasa tensile modulus in the range from 0.25 to 0.45 MPa.
 4. A compositesheet according to claim 1 wherein the adherent layer has a percentageelongation to break in the range from 300% to 10,000%.
 5. A compositesheet according to claim 1 wherein the adherent layer comprises astyrene-butadiene copolymer.
 6. A composite sheet according to claim 5wherein a molar ratio of styrene to butadiene in the styrene-butadienecopolymer is in the range from 0.1 to 10:1.
 7. A composite sheetaccording to claim 6 wherein the molar ratio is in the range of from 0.5to 3.1.
 8. A composite sheet according to claim 1 wherein the layermineral comprises platelets of a film-forming, 2:1 phyllosilicate.
 9. Acomposite sheet according to claim 8 wherein the 2:1 phyllosilicatecomprises vermiculite.
 10. A composite sheet according to claim 9wherein greater than 50% of the vermiculite has a particle size in therange of 0.5 to 5.0 microns.
 11. A composite sheet according to claim 10wherein from 70 to 95% of the vermiculite platelets has a particle sizein the range of 0.5 to 5.0 microns.
 12. A composite sheet according toclaim 9 wherein 80 to 99.9% of the vermiculite has a particle size inthe range of 0.1 to 5.0 microns.
 13. A composite sheet according toclaim 1 wherein the coating layer additionally comprises a cross-linkingagent.
 14. A composite sheet according to claim 13 wherein thecross-linking agent has a molecular weight of less than 5,000.
 15. Acomposite sheet according to claim 14 wherein the cross-linking agent isthe condensation product of melamine with formaldehyde.
 16. A compositesheet according to claim 1 wherein an additional heat-sealable layer ispresent on the surface of the adherent layer, remote from the coatinglayer.
 17. A composite sheet according to claim 1 wherein the substratelayer is a polymeric film.
 18. A composite sheet according to claim 1wherein the coating layer additionally comprises a copolymer of ethyleneand vinyl acetate.
 19. A method of producing a composite sheet byforming a substrate layer, applying to at least one surface thereof acoating layer comprising a layer mineral, and applying to the remotesurface of said coating layer a flexible adherent layer having apercentage elongation to break of greater than 150% and a tensilemodulus (1% secant modulus) of less than 2.0 MPa.
 20. A method accordingto claim 19 wherein the coating layer is applied to the substrate layerat a dry coat weight in the range of 0.25 to 50 mg/dm².